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University entomologists in the Mid-South and scientists at Syngenta are continuing to investigate resistance/tolerance in tobacco thrips to neonicotinoid-based seed treatments containing thiamethoxam.

Researchers are also studying the interaction between seed treatments and pre-emerge herbicides as a contributing factor in reduced efficacy in thrips control.

An annual survey of entomologists in the Cotton Belt, Cotton Insect Losses, indicates a rise in control costs and yield loss to thrips over the last few years in the Mid-South. In some states, losses have been precipitous. Cotton bales lost to thrips in Tennessee rose from 5,824 bales in 2009 to 42,508 bales in 2011. In 2013, Tennessee lost 21,189 bales to thrips.

Producers say they have had to make more foliar applications directed at thrips than they used to, and their rising costs of control bear that out. According to the losses report, the cost of controlling thrips has risen in every Mid-South state since 2006. The biggest increases are in Mississippi, which increased from $1.54 per acre in 2009 to $17.12 in 2013, and Missouri, where costs rose from $2.30 per acre to $7.96 over the same time period.

Mid-South university entomologists, weed scientists and Syngenta, which markets several seed treatments containing thiamethoxam, are conducting studies to help them better understand the interaction between seed treatments, weed resistance, plant health and to determine the scope of resistance or tolerance issues in thrips, which at this time appears limited to areas in the Mid-South.

They know that several factors are in play. Insecticidal seed treatments are now used on over 95 percent of cotton fields in the Mid-South. At the same time, the use of pre-emergence herbicides has drastically increased across the region due to the proliferation of glyphosate-resistant Palmer amaranth.

Scientists agree that pre-emerges are probably having some effect on the cotton plant which may lead to other problems. “Pre-emerges do slow the plant down, and anything that slows the plant growth is going to open the door to pest problems like thrips and disease,” said University of Arkansas Extension entomologist Gus Lorenz. “But I think the overriding factor is this tolerance issue.”

Assays run on thrips collected by Arkansas entomologists last year. “indicated that there is a resistance/tolerance right now,” Lorenz said. “The preliminary study backed up what we had been observing for the last couple of years.”

Scott Stewart, IPM Extension specialist at the University of Tennessee said Tennessee studies on thrips “show that thiamethoxam did not perform as well as imidacloprid in controlling thrips. That’s probably the driving factor behind this poor performance.”

University of Tennessee studies indicated that herbicides negatively affected the performance of at-planting thrips treatments, primarily with thiamethoxam, probably due to the colder climes of west Tennessee where cotton can often remain susceptible to thrips beyond the time period when it is typically protected by seed treatments.

“I don’t think there’s any question, now that we’ve gone back to the world of pre-emerge herbicides, sometimes we cause a little more injury and delay growth than we like to,” Stewart said.

Late last spring, Syngenta collected thrips samples from four suspect populations in the Mid-South, one from each of the states of Louisiana, Arkansas, Mississippi and Tennessee.

“We did find a reduced susceptibility in the thrips populations compared to a known susceptible population that we got from North Carolina State University,” said Scott Martin, Syngenta’s technical produce lead, seed care.

“This year we’re trying to be a lot more proactive,” said Martin, who believes the reduced efficacy is limited to the Mid-South. “We’ve been out the last couple of weeks collecting thrips all across the cotton geography, excluding the western states, from Texas all the way up to Virginia.”

Martin agrees other factors could be contributing to reduced efficacy, such as the increased use of pre-emerge herbicides in cotton fields where producers are trying to control glyphosate-resistant Palmer amaranth.

“Anything that slows the cotton plant down is going to change the way thrips interact with it. Is it the major factor of what’s going on? I don’t think so, but that’s just my personal opinion at this time.”

Stewart says that many Tennessee cotton producers have already switched to imidacloprid-based seed treatments, but he is concerned that imidacloprid could eventually be affected too. “We’re recommending growers use imidacloprid, and that they consider making a 1- to 2-leaf automatic foliar application for thrips, which is something we do commonly anyway. I think now, given the questions we have, we need to do that proactively. The thing you can’t do with thrips is get behind. That’s why we use at planting treatments.”

Arkansas is recommending that growers use an imidacloprid-based product for seed treatments. “But even if they are using imidacloprid seed treatments, they still need to watch their cotton very closely when it comes out of the ground,” Lorenz said. “Be prepared to make foliar applications as needed. We’re just not getting the level of control that we’ve had in the past.”

Lorenz suggests foliar sprays such as Radiant, “which is a different class of chemistry, or Orthene or Bidrin. As long as we stick to those, we will be fine. What we what we need to avoid is pyrethroids over the top of seedling cotton to avoid flaring problems with mites and aphids.”

Syngenta is recommending that producers in Mid-South cotton-growing states using neonicotinoid seed treatments in cotton “either over-treat with another seed-applied insecticide such as Orthene, or be very aggressive in over-spraying for thrips,” Martin said.

Over-spraying for thrips is more common in the Southeast cotton-growing region, Martin says, and could explain why the region has not observed reduced efficacy in a neonicotinoid.

Syngenta hopes to find answers soon, Martin says. “We’re trying to support the University entomologists. We have been very open about the methodology we are using, and trying to let everyone share in that methodology so our research results can be compared across different projects. Neonicotinoids are an important insecticide chemistry, and we want to do everything that we can to preserve them.”